1. Field of the Invention
The present invention relates to a data storage and retrieval system. More particularly the present invention relates to a robotic system for storing and retrieving large amounts of data and to a robot for use in such a system.
2. Discussion of the Related Art
It is a constant aim in data processing to provide means for data storage which is inexpensive and readily accessible to an operator of the data processing system. Storing very large amounts of data in main memory microchips is very expensive. Data may also be stored on peripheral storage devices which include magnetic tape storage devices, magnetic direct access storage devices (DASD) and optical storage devices. In some applications the amount of information which needs to be retained is vast and numerous disks or tapes are required for storing it. Such tapes and disks have, in the past, been stored in libraries and loaded into peripheral storage devices manually before being accessed by a data processing system. This of course has an undesirable impact on the amount of time taken for a data processing system to access data stored in the library and furthermore, human labor is expensive.
Automated storage libraries have been developed to manage the storage of large amounts of tapes or disks with human intervention only being required for maintenance and fault fixing. An automated storage library is usually a memory subsystem of a larger data processing system. An automated storage library usually includes: a plurality of storage shelves containing cells or slots for retaining data storage media such as magnetic tapes, magnetic disks or optical disks; a robotic picker mechanism; and one or more peripheral storage devices for transferring data between the magnetic storage media and the main data processing system. When data is required by the data processing system the robotic picker is instructed to retrieve a data storage medium and load it into a peripheral storage device so that data may then be accessed by the data processing system. The data storage media are usually stored in a cassette or cartridge so that they are more easily handled and less likely damaged by the robotic picker mechanism. Automated storage libraries are usually enclosed within secure housing to reduce contamination on the storage media and prevent personnel from inadvertently interfering with the robot picker mechanism.
An example of an automated storage library is the 3850 Mass Storage Subsystem for the storage and retrieval of magnetic tape modules which was introduced in the 1970's by IBM Corporation. More recently, automated storage libraries for magnetic disks and optical disks have been introduced. Modern data processing has reduced the amount of time taken to transfer data from a peripheral storage device to the central processor of the data processor. The time taken to fetch a data storage medium from a storage cell and load it into a peripheral storage device is limited by the capabilities of the robotic picker mechanism.
US-A-4,907,889 describes a video cassette library retrieval and sequencing system. In this system the robot picker mechanism is secured to the floor between a video cassette machine and a rotary cassette carousel which holds the video cassettes. The rotary cassette carousel is arranged so that each cassette slot can be brought into a position which is accessible by the picker mechanism. The picker mechanism includes a pair of angularly offset cassette gripper elements which can be operated independently of each other and are mounted on a cassette transfer arm which is itself mounted on the central column of the robot. The pair of gripper elements are mounted on the cassette transfer arm for rotary movement along a predetermined path. Although the robot's retrieval time is favourable because of its limited movement, a disadvantage of this system is that the storage capacity is limited to the number of video cassettes which can be stored on the carousel. In the system described, the storage capacity could be increased by the addition of another rotary cassette carousel but nevertheless the storage capacity is limited to the number of video cassettes which can be stored within reach of the static robot picker mechanism.
An example of an automated storage library in which the storage capacity is not limited by movement of the robotic picker is described in DE-3918198-C1. In the tape archive described, the cassette tapes are stored in a plurality of aligned rotating storage carousels. The tape cassettes are read from tape reader units located near the end of the line of storage carousels or on either side of the line of carousels. The cassettes are delivered to the tape reader units by a pair of robotic pickers, one picker operating on each side of the line of storage carousels. Each robotic picker has a predefined action area which does not overlap with that of the other picker. Each of the rotating carousels can be brought into an access position for the first robot and the second robot. A disadvantage of this library is the expense of providing and maintaining two robots.